1. Field of the Invention
The present invention relates to an image forming apparatus which ejects ink for recording an image on a recording medium and a control method of the same.
2. Description of the Related Art
Generally, image forming apparatuses including a recording head that drives an actuator comprised of a piezoelectric element or a heater element for ejecting ink, what are called ink-jet printers are known. When an actuator is driven, heat is generated by both of the actuator itself and a drive section (driving IC) of the actuator. In recent image forming apparatuses, there is a trend wherein a heat generation amount increases with an increase in number of nozzles due to speed up of recording rate and an adoption of a line head. In order to suppress heating of the recording head arising from the heat generation, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-199021 discloses an art wherein a discharge path (ink return path) is provided to an ink supply path including a recording head for circulating ink. This art enables heat to be removed from the recording head by the circulation of ink, and thereby suppressing heating of the recording head.
Fluids, such as ink has a characteristic of decreasing in viscosity as its temperature rises. In image forming apparatuses too, viscosity of ink decreases as ink temperature rises, regardless of types of ink used. Because of the above characteristic, amounts of ink to be discharged (drop amount) differ between a case where ink temperature is high and a case where ink temperature is low, even if the recording head is driven with the same drive voltage in the both cases. More specifically, an amount of ink to be discharged is increased when ink temperature is high. Therefore, ink temperature inside a recording head rises when the driven recording head is heated, and quality of images to be formed (or printing quality) may not be kept constant when the recording head is driven with the same drive voltage as in the case where the ink temperature is low. Under the circumstances, a head drive voltage needs to be corrected. For instance, according to Jpn. Pat. Appln. KOKAI Publication No. 2006-199021, a temperature sensor is provided downstream of a recording head in an ink circulation path for cooling the recording head. The temperature sensor detects ink temperature, and a drive voltage of the recording head is corrected on the basis of a detected value. Furthermore, another temperature sensor is provided upstream of the recording head in the ink circulation path for regulating temperature of ink supplied to the recording head.
Conventional temperature characteristic graphs of FIG. 9 indicate examples wherein temperature of ink downstream of a recording head in an ink circulation path is detected. This figure shows a difference between example A wherein ink temperature is high upstream of the head and example B wherein ink temperature is low upstream of the head. In the graphs, TC indicates ink temperature in a nozzle section when ink temperature upstream of the head is high, and TD indicates ink temperature in the nozzle section when ink temperature upstream of the head is low. It is assumed that the nozzle section exists substantially in the middle of an interval starting from a point when ink flows into the recording head to a point when the ink flows out therefrom.
It cannot be known what degrees the ink temperature was when the ink flew in merely by detecting temperature of ink downstream of the head. In other words, it cannot be known what degrees the temperature of ink rose by going through the head. Surrounding temperature of nozzles ejecting ink of the recording head is reflected in a difference between ink temperature upstream of the head and ink temperature downstream of the head. The surrounding temperature of nozzles ejecting ink of the recording head therefore cannot be known unless the temperature difference is obtained.
Also, as shown in FIG. 9, ink temperatures TC and TD in the nozzle section depend on ink temperature upstream of the head. It is therefore unreasonable to estimate surrounding temperature of nozzles ejecting ink of the recording head and surrounding temperature of nozzles ejecting ink merely from temperature of ink downstream of the head. Correcting a drive voltage of the recording head by using only ink temperature downstream of the head causes a large margin of error between a corrected drive voltage and an appropriate drive voltage.